Bear in mind though that an "atomic rename" isn't really atomic under most filesystems

rename is atomic on POSIX systems. Win32 has atomic rename and I just checked and rename uses it on modern Win32 operating systems. That qualifies as "most" of the Perl universe in my book (covering the two most common Perl environments, even if TheDamian chooses to call one of the top two "obscure"). Perhaps you have evidence to the contrary or perhaps you are thinking of pre-rename methods using link/unlink?